1. Field of the Invention
The invention is concerned with magnetic "bubble" devices. In particular, the invention is concerned with devices which include a supported layer of magnetic garnet material, generally, but not necessarily, on a non-magnetic garnet substrate. Such devices depend for their operation on nucleation and/or propagation of small enclosed magnetic domains of polarization opposite to that of the immediately surrounding material in the supported layer. Functions which may be performed include switching, memory, logic, etc.
2. Description of the Prior Art
For a number of years, there has been widespread interest in a class of memory or switching elements known as "bubble" devices. The term "bubble" is descriptive of the generally cylindrical form taken by the single wall domains, presence or absence of which constitutes the memory bits essential to operation. Such single wall domains, which may assume a variety of configurations, represent localized regions of one magnetic polarization within a surround of opposite polarization. Polarization, in either case, is largely orthogonal to a major surface of the device so that domains may be described as emergent--that is, with polarization "emerging" from a major plane. There is a vast body of literature on devices of this category. See, for example, Vol. MAG-5, IEEE Transactions on Magnetics page 544 (1969) and Scientific American, June (1971) p. 78-90.
Material requirements imposed on magnetic compositions have, in many respects, been more stringent than those imposed by other devices. For example, contemplation of domain or bit size of the order of a micrometer or less has carried with it the attendant requirement that material imperfections affecting nucleation or propagation be of a smaller size scale. Requirements on uniformity, both physical and compositional, have been legend, and solutions to these many problems have been impressive. Technology has resulted in development: of supposedly centrosymmetric garnets evidencing controllable and pronounced, unique easy directions of magnetization; of procedures for growing epitaxial layers of perhaps the highest physical and compositional uniformity yet seen under growth conditions considered a marked departure from all prior techniques; and of ancillary advances, e.g., concerned with fine scale access circuitry, lithographic techniques, etc. The program has already had and will continue to have widespread implications in a variety of arts.
It has been recognized for some time that a major material problem involves the precise manner in which the emergent domain is produced. Since garnet materials have been the leading contenders for bubble devices for some time, concern over emergence has generally been in terms of such materials. Two major approaches have been followed: the first, "growth induced anisotropy" relies on mixed population in a given crystallographic site, usually the dodecahedral site. Such mixed population of appropriate ions results in some form of local strain or preferential ordering attendant upon growth. Growth-induced unique easy direction is maintained at all but extremely high temperature (temperatures not ordinarily contemplated during use). Magnetic properties in growth-induced materials may, in selected compositions, be substantially temperature independent or may vary so as to match bubble properties to temperature in a desired manner. Characteristically, such compositions include praseodymium, neodymium, samarium, europium or terbium together with different rare earth (or yttrium) ion. Growth induced materials are eminently useful for many device designs.
A second approach makes use of massive strain ordinarily induced by a disparity between crystallographic lattice dimensions of supported layer and substrate. For example, supported epitaxial materials evidencing a negative value of magnetostriction, when supported on a substrate material of larger lattice dimension, show the emergent domain behavior. Best "strain induced" materials have, to date, had the advantage of somewhat increased domain wall mobility as contrasted with the growth induced material but have shown the disadvantage of significant temperature dependence of magnetic properties of concern in device operation.
Regardless of the mechanism/s responsible for the necessarily unique easy direction of magnetization, a characteristic of device concern is permitted speed of record and access. This parameter is ultimately dependent on the rate with which the bubble may be moved from any given position to an adjacent position. It, therefore, depends on such considerations as device design, traversal distance, and characteristics of auxiliary equipment, such as drive frequency.
Materials within which bubbles are nucleated and/or propagated are characterized by an inherent speed factor: mobility. This factor, when multiplied by the field in the material resulting from the applied drive field, results in a "velocity" term. In many of the materials used in the earlier stages of bubble devices, record and access time were limited by mobility. So, for example, materials containing substantial amounts of terbium in the dodecanedral site were typically characterized by mobility values of about 100 cm/sec/Oersted. Applied drive fields, conveniently at a level of perhaps 5 Oersteds, in consequence, resulted in velocities of about 500 cm/sec. As materials with higher mobility were designed, it was found that real rates of record or access were limited by another consideration. So, Samarium-containing materials, characterized by mobilities as high as 1300 cm/sec/Oersted, while yield velocities as high as 3000 cm/sec with relatively low drive fields, resulted in a loss of information as drive field was increased. Velocity limits generally did not exceed 1 megahertz operation. This is the frequency of field reversal and, therefore, the cycle time for movement of any given bubble to an adjacent position. For structures with position spacings of approximately 28 .mu.m, this is equivalent to a velocity of 2900 cm/sec. Attempts to exceed such limiting velocities were found to result in loss of information. While the responsible mechanism has not been irrefutably established, it appears to involve a change in domain wall configuration, probably from a simple Bloch wall to a wall with a number of Block-to-Neel transitions. This characteristic, sometimes referred to as "dynamic conversion," results in bubbles which are "erratic" in that they do not follow the drive field in the predictable fashion of bubbles moving at lower velocities.